1. Field of the Invention
The present invention relates to an optical disk apparatus for reproducing a signal recorded in an optical disk optically using a light source such as a laser, and in particular to an optical disk apparatus for adjusting the balance based on tracking error signal by a tracking error detection system using a phase difference method, adjusting a control target position in focusing control, and adjusting a gain in the focusing control and tracking control.
2. Description of the Related Art
Optical disk apparatuses (hereinafter, referred to also as "optical reproduction apparatuses") for reproducing a signal from a medium having video information, computer data or the like recorded therein have recently been demanded to have a higher speed in data reading and a higher level of reliability.
With reference to FIGS. 10 through 12, a conventional optical reproduction system will be described.
FIG. 10 is a diagram illustrating a structure of a conventional optical disk apparatus. A light beam emitted by a light source 101 such as a semiconductor laser is collimated by a collimator lens 102, reflected by a polarization beam splitter 103, passed through a 1/4-wave plate 104, converged by a converging lens 105, and radiated toward an optical disk 107 which is rotated by a motor 111. The light reflected by the optical disk (reflected light) is passed through the converging lens 105, the 1/4-wave plate 104, the polarization beam splitter 103 and a collection lens 108, and then radiated toward a light detector 109 divided into four light receiving areas.
The converging lens 105 is attached to a movable section of an actuator 106. When an electric current flows through a focusing coil of the actuator 106, the converging lens 105 moves in a direction perpendicular to the surface of the optical disk 107; and when an electric current flows through a tracking coil of the actuator 106, the converging lens 105 moves in a radial direction of the optical disk 107. Hereinafter, the area where the converging lens 105 is movable is referred to as the "lens moving area".
The actuator 106, the 1/4-wave plate 104, the polarization beam splitter 103, the collimator lens 102, the light source 101, the collection lens 108, and the light detector 109 divided into four light receiving areas 109 are included in a head unit 110.
Outputs from four light receiving areas 109a, 109b, 109c and 109d are respectively passed through amplifiers 113a, 113b, 113c and 113d and then input to a focusing error circuit 114. The focusing error circuit 114 outputs a focusing error signal which indicates the positional offset between the focal point of the optical beam and an information recording face of the optical disk 107, based on the signals from the amplifiers 113a, 113b, 113c and 113d. The focusing error signal is processed by an adder 115, a variable amplifier 117, a phase correction device 118 for correcting a phase difference, and a driving circuit 119 for amplifying the power, and then applied to the focusing coil of the actuator 106. The head unit 110 is controlled so that the converging point of the light beam is positioned on the information recording face of the optical disk 107.
The outputs from the four light receiving areas 109a, 109b, 109c and 109d are input to a phase difference tracking error circuit 120 respectively via the amplifiers 113a, 113b, 113c and 113d. The phase difference tracking error circuit 120 outputs a tracking error signal which indicates the positional offset between the focal point of the optical beam and a track of the optical disk 107, based on the phase difference information from the amplifiers 113a, 113b, 113c and 113d, the phase difference information being obtained when the light beam passes through the pit in the track of the optical disk 107. The phase difference tracking error circuit 120 adjusts the balance based on the tracking error signal based on the signal from a controller 150.
FIGS. 11A through 11C show the principle by which the phase difference tracking error signal is detected. When a beam spot 107a passes above a pit 107b which are included in a track of the optical disk 107, the intensity pattern of the reflected light of the beam spot 107a changes over time. Specifically, as shown in FIG. 11B, when the beam spot 107a passes through the center of the pit 107b, i.e., the center of the track, the intensity pattern of the reflected light of the beam spot 107a changes symmetrically with respect to the center of the track.
As shown in FIG. 11C, when the beam spot 107a passes left with respect to the center of the pit 107b, the intensity pattern of the reflected light of the beam spot 107a changes so that the positions of the more intense part of the reflected light moves counterclockwise. As shown in FIG. 11A, when the beam spot 107a passes right with respect to the center of the pit 107b, the intensity pattern of the reflected light of the beam spot 107a changes so that the positions of the more intense part of the reflected light moves clockwise. As the beam spot 107a becomes further from the center of the pit 107b, the contrast of the intensity pattern of the reflected light of the beam spot 107a becomes clearer.
A method for detecting a tracking error signal utilizing such a change in the intensity pattern of the reflected light of the beam spot 107a is referred to as the "phase difference method". In other words, according to the phase difference method, the phases of the two signals obtained from two light receiving areas of the four light receiving areas of the light detector 109 which are located diagonally opposite to each other are compared, and the positional offset between the beam spot and the track is detected based on the advance or delay in the phase.
FIG. 12 shows an internal configuration of the phase difference tracking error circuit 120. The phase difference tracking error circuit 120 outputs a tracking error signal based on the outputs from the light receiving areas 109a and 109b and the outputs from the light receiving areas 109c and 109d. The set of light receiving areas 109a and 109b is bordered from the set of light receiving areas 109c and 109d by a border running perpendicular to a direction 109e in which the pit 107b proceeds, i.e., the tangent direction of the track.
The outputs from one of the above-mentioned two sets of light receiving areas, e.g., the outputs from the light receiving areas 109a and 109b are respectively input to phase correction sections 120a and 120b via the amplifiers 113a and 113b and delayed or advanced by the phase correction sections 120a and 120b.
Adders 120c and 120d each add the output signals from the light receiving areas of the light detector 109 diagonally opposite to each other. Specifically, the adder 120c adds the output signal from the light receiving area 109a which has been corrected by the phase correction section 120a and the output signal from the light receiving area 109c without being corrected. The adder 120d adds the output signal from the light receiving area 109b which has been corrected by the phase correction section 120b and the output signal from the light receiving area 109d without being corrected. A comparator 120e outputs a tracking error signal which indicates the positional offset between the focal point of the light beam and the track, based on the phase difference between the output from the adder 120c and the output from the adder 120d.
With reference to FIG. 10 again, the tracking error signal is applied to the tracking coil of the actuator 106 via an adder 122, a variable amplifier 124, a phase correction device 125 for correcting a phase difference, a switch 126 for turning on or off the tracking control based on the output from the controller 150, and a driving circuit 127 for amplifying the power. Thus, feedback control of the converging lens 105 is performed so that the focal point of the light beam is positioned on the track.
A driving circuit 135 is provided for controlling the motor 111 based on the output from the controller 150. The controller 150 controls the motor 111 so that the rotation rate of the motor 111 is a prescribed rate.
The gain in the focusing control and tracking control disperses due to, for example, the dispersion in the dimensions in optical disks, the over-time change in the dimensions of the optical disk, and the dispersion in the dimensions in optical heads. As the gain decreases excessively, the control precision deteriorates. As the gain increases excessively, the control system becomes unstable and may sometimes oscillate.
When the focusing control causes an offset, the quality of a reproduction signal deteriorates and thus the reliability for reading signals declines.
In order to avoid these inconveniences, the following adjustment is performed in the conventional optical disk apparatus.
First, the gain adjustment in the focusing control system will be described.
An external disturbance generator 132 outputs an external disturbance signal in response to an instruction signal from the controller 150. The external disturbance signal is added to the focusing error signal by the adder 115 and thus added to the focusing control system. The focusing error circuit 114 outputs a signal from the focusing error system in response to the external disturbance signal. A focusing error measuring circuit 136 extracts the response signal to the external disturbance signal which is included in the output signal from the focusing error circuit 114, and outputs the signal to the controller 150. The controller 150 compares the external disturbance signal generated by the external disturbance generator 132 and the response signal to the external disturbance signal from the focusing error measuring circuit 136, and determines the amplification ratio of the variable amplifier 117 so that both signals have a prescribed relationship.
Next, the gain adjustment in the tracking control system will be described.
As in the gain adjustment in the focusing control system, an external disturbance generator 134 outputs an external disturbance signal in response to an instruction signal from the controller 150. The external disturbance signal is added to the tracking error signal by the adder 122 and thus added to the tracking control system. The tracking error circuit 120 outputs a signal from the tracking error system in response to the external disturbance signal. A tracking error measuring circuit 133 extracts the response signal to the external disturbance signal which is included in the output signal from the tracking error circuit 120, and outputs the signal to the controller 150. The controller 150 compares the external disturbance signal generated by the external disturbance generator 134 and the response signal to the external disturbance signal from the tracking error measuring circuit 133, and determines the amplification ratio of the variable amplifier 124 so that both signals have a prescribed relationship.
Offset adjustment in the focusing control system will be described.
The offset adjustment in the focusing control system is performed by measuring the jitter of a reproduction signal. An adder 128 outputs a signal (RF signal) to a jitter amount detection circuit 129. The signal (RF signal) sent to the jitter amount detection circuit 129 is the sum of the signals from the light receiving areas 109a, 109b, 109c and 109d of the light detector 109 which have been processed by the amplifiers 113a, 113b, 113c and 113d. The jitter amount detection circuit 129 measures the jitter amount of the RF signal. The controller 150 outputs an offset signal to the adder 115 so as to minimize the jitter amount measured by the jitter amount detection circuit 129.
The conventional optical disk apparatus described above has the following problems.
When the converging lens 105 moves in a radial direction of the optical disk 107 (lens shift), an offset occurs in the tracking error signal detected by the phase difference method. The offset amount varies in accordance with the de-focusing state or the depth of the pit in the optical disk 107. When the tracking control is operated in the state where the offset occurs in the tracking error signal by the lens shift, if the decentering of the optical disk 107 is excessively large, a large offset occurs in the tracking error signal. Thus, the tracking offset increases, which deteriorates the tracking control precision. Since the tracking control thus becomes unstable, the reliability of the optical disk apparatus declines.
When the focusing position is offset excessively, the quality of the RF signal deteriorates. Accordingly, the jitter amount cannot be measured by the jitter amount detection circuit 129. As a result, it becomes difficult to adjust the focusing position of the converging lens 105 at a prescribed target position.
When the optical disk apparatus receives a vibration or a scratch exists on the optical disk 107 during the adjustment of the focusing position of the converging lens 105, the amplitude of the reproduction signal or the jitter amount fluctuates. Accordingly, the adjustment precision of the focusing position of the converging lens 105 declines. When a further vibration or an external disturbance caused by a scratch on the optical disk 107 is added when the target position of the focusing position is changed, the focal point of the light beam converged by the converging lens 105 cannot be within a prescribed area (in this specification, such a phenomenon is referred to as "focusing skip").
When the optical disk apparatus receives a vibration or a scratch exists on the optical disk 107 during the gain adjustment of the focusing control system or the tracking control system, a further vibration or an external disturbance caused by the scratch on the optical disk 107 is added to the external disturbance which is already added to the control system in order to adjust the gain. Accordingly, focusing skip or tracking skip occurs (in this specification, "tracking skip" refers to that the light beam converged by the converging lens 105 is not positioned on a target track of the optical disk 107.)
In the case of an optical disk apparatus for reproducing a compact disk at a plurality of rates of standard, 2.times., and 6.times., various adjustments such as gain adjustment are performed at the start of the optical disk apparatus at the respective reproduction rate. Accordingly, the start-up time of the optical disk apparatus at the respective rate is extended.